In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Multislice CT systems are used to obtain data for an increased number of slices during a scan. Known multislice systems typically include detectors generally known as 3-D detectors. With such 3-D detectors, a plurality of detector elements form separate channels arranged in columns and rows. Each row of detectors forms a separate slice. For example, a two-slice detector has two rows of detector elements, and a four-slice detector has four rows of detector elements. During a multislice scan, multiple rows of detector cells are simultaneously impinged by the x-ray beam, and therefore data for several slices is obtained.
In a CT system having such a 3-D detector, the intensity of detector measurements are derived by combining, along the z direction, multiple detector outputs. These outputs are supplied as inputs to a data acquisition system. If the detector outputs to be combined are obtained from detectors having different individual gains, the combined signal represents a weighted sum of the incoming detector signals where the different detector gains cause different weighting. Know gain calibration algorithms may be used to correct for this different weighting. However, these known calibration algorithms are unable to correct the signals when the error is caused by z-axis non-uniformity of the x-ray beam and the detector. As a result, a small variation in the non-uniformity of the z-response results in a substantial amount of artifacts in the reconstructed image.
It would be desirable to correct for z-axis non-uniformity of the x-ray beam to reduce image artifacts. It also would be desirable to more accurately generate an image regardless of the z-axis non-uniformity of the detector arrays. It further would be desirable to provide such imaging without significantly increasing the cost of the system.